388 Rohb on Superficial Deposits in Canada. 



been precipitating in the channel flowing from it, a very large in- 

 crease of coloring matter, and of a deep clear red color." 



The inference which I would draw from all the circumstances I 

 have so miuutely detailed is, that powerful chemical reactions are 

 at the place referred to, in constant operation at the surface or in 

 fissures of the rocks underlying the superficial deposits, decom- 

 posing the pyrites and converting it into the sulphate of protoxide 

 of iron, which is soluble in water and is brought to the surface by 

 the springs, and then receiving oxygen from the atmosphere, and 

 being afi'ected by the alkaline solutions also contained in the water, 

 is again changed with the insoluble peroxide and precipitated on 

 the surface of the ground, forming an ochre bed. The abstraction 

 of the material thus displaced will, in the long process of these 

 operations, leave cavities under the surface which must be filled 

 by the subsidence of the superincumbent earth in the same man- 

 ner as is often observed in places where coal beds have been 

 entirely excavated and abandoned. I have little doubt that the 

 remarkable dell or ravine which I have described is of this nature 

 and origin ; and that the series of isolated pits or depressions 

 ranging in the same line with it is only a continuation of it, but at 

 a less advanced stage of formation. 



To account for the oxidation of the pyrites, which I have sup- 

 posed to be the primary cause of all these phenomena, is not such 

 an easy matter, unless we can conceive it to exist in the bowels of 

 the earth in a minutely subdivided state. 



" It is well known," says Sir Charles Lyell, " that mixtures of 

 sulphur and iron sunk in the ground and exposed to moisture give 

 out sufficient heat to pass gradually into a state of combustion, 

 and to set fire to any bodies that are near. If a large quantity 

 of clean iron filings be mixed with a still larger proportion of sul- 

 phur, and as much water as is necessary to make them into a firm 

 paste, let the mixture be then buried in the earth and the soil 

 pressed firmly down upon it. In a few hours it will grow warm, 

 and swell so as to raise the ground. Sulphurous vapors will 

 make their way through the crevices and sometimes flames appear. 

 There is rarely an explosion, but when this happens the fire is 

 vivid and if the quantity of materials is considerable, the heat and 

 fire both continue for a long time." 



The spontaneous combustion of beds of bituminous shale and 

 of the refuse coal thrown out of mines is also generally due to 

 the decomposition of pyrites — and it is the contact of water, 



